ESTRO 2024 - Abstract Book

S1541

Clinical - Lower GI

ESTRO 2024

a five-year follow-up of this trial confirmed the reduction in disease-related treatment failure and distant metastases. However, there was a noticeable rise in locoregional recurrence compared to long-course chemoradiotherapy (2). In our research, we have explored a dose escalation strategy up to 5 x 6 Gy targeting the gross tumor volume (GTV) using MRI-guided radiotherapy, followed by chemotherapy according the RAPIDO approach. The objective of our study is to evaluate the effectiveness of this approach in a retrospective cohort of patients, with one year of follow-up data.

Material/Methods:

Between July 2021 and October 2022, a total of 21 patients diagnosed with locally advanced rectal cancer underwent radiotherapeutic treatment on a MRIdian system (ViewRay, Inc.), with a one-year follow-up available at the time of writing. Patients in this retrospective study received 5 x 5 Gy on the mesorectum with a simulated integrated boost (SIB) on the GTV till 5 x 6 Gy using online adaptive MRI-guided radiotherapy with tumor gating. Afterwards chemotherapy was administered using CAPOX (6 cycles) or FOLFOX (9 cycles). In case of complete clinical remission (after about 6 months), the patient and physician together decided whether or not a watchful waiting approach was considered, with a planned follow-up every 3 months. For the evaluation of the effectiveness of our new RAPIDO-inspired approach, the number of patients having no relapse within 9 months and 1 year period after the start of radiotherapy was considered as primary outcome. This analysis is part of a clinical study that received approval from the ethical committee of UZ Brussel (EC-2021-135). Of the 21 patients who were in follow-up after more than one year, 8 patients (38%) had a complete clinical remission after total neoadjuvant treatment leading to a watchful waiting strategy. In the remaining 13 patients (62%) the tumor was surgically removed at the end of neoadjuvant treatment via total mesorectal excision (TME), via partial mesorectal excision (PME) or via abdominoperineal resection (APR). Of these 13 patients, anathomopathological reports of the tumor tissue showed Dworak regression scores of 2, 3, 4 (complete remission) for respectively 3, 5, 4 patients. After one year follow-up, 6 of the original 21 patients (29%) showed disease progression. Two patients in the watchful waiting group relapsed locally after 9 months. In the surgery group, 2 patients relapsed metastatically after 9 months and 2 after one year. The one year progression free survival was 75% in the watchful waiting group (6 out of 8 patients) and 69% in the surgical group (9 out of 13). Results:

Conclusion:

MRI-guided total neoadjuvant treatment in locally advanced rectal cancer with application of a simultaneous integrated boost on the GTV resulted in a high clinical and pathological complete remission of 57% at the end of radiochemotherapy. After one year follow-up, 71% of all patients were progression free.

Keywords: total neoadjuvant treatment, rectal cancer, RAPIDO

References:

(1) Bahadoer RR, Dijkstra EA, van Etten B, et al.; RAPIDO collaborative investigators. Short-course radiotherapy followed by chemotherapy before total mesorectal excision (TME) versus preoperative chemoradiotherapy, TME, and optional adjuvant chemotherapy in locally advanced rectal cancer (RAPIDO): a randomised, open-label, phase 3 trial. Lancet Oncol. 2021 Jan;22(1):29-42. doi: 10.1016/S1470-2045(20)30555-6.